NEW YORK (Fortune) -- Most of the analysis of Microsoft's $45 billion hostile offer for Yahoo has focused on technology and the role Yahoo could play in the Microsoft-Google wars. Today, though, let's look at what almost everyone has overlooked:
the numbers. More specifically, we'll look at the way that one of Wall Street's biggest hitters, Joe Rosenberg, looks at the proposed deal.

Rosenberg, chief investment strategist at Loews Corp. (LTR, Fortune 500), the giant conglomerate, is one of the most influential voices on
Wall Street. In fact, his criticism of Microsoft in a Barron's interview two years ago, in which
he criticized the firm's use of capital and suggested a $50 billion stock buyback, was a big factor in sparking the $40 billion buyback program that Microsoft (MSFT, Fortune 500) launched in the fall of 2006.

So Rosenberg's opinion matters. And he thinks Microsoft's offer for Yahoo (YHOO, Fortune 500) is nuts. "This is like a person who's completely lost his mind,"
Rosenberg told me in an interview. "It's absurd. They're not going to earn anything like a reasonable rate of return on their investment in Yahoo. It just doesn't make sense."

"This deal would do more harm to Microsoft shareholders than any of its competitors can do to it," Rosenberg said. "The company has lost sight of its principal focus, which is to
produce value for shareholders."

“This deal would do more harm to Microsoft shareholders than any of its competitors can do to it.”

Joe Rosenberg

Before we proceed, two disclosures. First, Rosenberg told me that Loews owns Microsoft stock. Second, Loews is one of my biggest individual holdings, which means I have money riding
on Rosenberg's investment acumen.

Back to the main event. Until the Yahoo bid surfaced, Rosenberg was predicting that Microsoft would earn almost $2 a share in its current fiscal year, rising steadily to more than $4
a share in four years. (A major element in his thinking: Microsoft's sales to "developing markets" such as China, India and Eastern Europe will soar.) So at $32 - its price before
news of the Yahoo offer drove its stock down sharply Friday - Microsoft looked cheap to Rosenberg. Now, he fears, Microsoft may be making the same mistake as other companies that did
large, failed high-tech takeovers.

No, he's not talking about the 2000 deal that combined America Online with my employer Time Warner (TWX, Fortune 500), which many people have cited as a cautionary tale for
Microsoft-Yahoo. Wrongly, in Rosenberg's opinion (and mine). How so? Because America Online's purchase of Time Warner turned out great for AOL shareholders, whose stock fell far less
than other Internet issues when the bubble popped. The deal was disastrous for the sellers - the old Time Warner's stockholders - because the value of their shares was eviscerated.

The real parallels to Microsoft-Yahoo, says Rosenberg, citing his 47 years on Wall Street, are two largely-forgotten disasters: Xerox' $1 billion purchase of Scientific Data Systems
in 1969 and AT&T's (T, Fortune 500) $7.5 billion purchase of NCR in 1991. In both cases, the acquiring
company paid top dollar for firms whose products and technology rapidly became obsolete.

Rosenberg doesn't pretend to be a tech maven, but he says it's clear that the market - which sent Yahoo's stock down 45% from October through last Thursday - is saying something
negative about Yahoo's businesses and prospects. Microsoft, Rosenberg says, would be well-advised to listen.

Should Microsoft buy Yahoo, Rosenberg says, he, like other folks looking at this deal, expects Microsoft and Google to engage in price wars in the search and advertising businesses.
But Rosenberg carries that thought one level further. Such a price war would hurt Yahoo's already-anemic profit margins, Rosenberg says, making a Microsoft purchase even more
problematic. Microsoft's future free cash flow per share would be substantially higher if it buys back its own shares, he said, rather than buying Yahoo by issuing about $23 billion
of new stock and spending a net $15 billion or so in cash. (That cash number takes into account the approximately $8 billion of cash and marketable securities that Yahoo owns.)

Rosenberg says he's not trying to hurt Microsoft, he's trying to help. "They don't realize that by criticizing this deal, I'm trying to do them a favor," he says. And, of course, to
do Loews a favor, too.